A lot of indie games that are cheap on Steam are well-worth the short time investment:

Dear Esther (if you're into that sort of thing, its only about an hour long and it isn't really a game by definition)
Bastion (great atmosphere, really cool story-telling mechanic)
Limbo (awesome atmosphere and good puzzles)

As for others:

Portal 1/2 (you can run through both of them in about 10 hours tops)
Mirror's Edge (also short)
Fallout 3 or NV if you have that kind of time on your hands
Deus Ex: Human Revolutions
Civilization V
Bioshock
Dead Space
Mass Effect